ENN552 Solar Thermal Systems
ENN552 Solar Thermal Systems
About 87% of the World energy demand is supplied by Fossil fuels only less than 13% of renewable sources of energy is used to supply the energy demand in world wide. Energy generation by fossil fuels creates a very negative impact on environmental health as the emission of harmful gases occurs due to burning these fuels. The production of electricity from the renewable sources are the most environmental friendly method as the renewable sources of energy are self-replenishing and inexhaustible. The Solar energy is a renewable source of energy, which is collected from the sunlight. There are many methods to generate electricity from the solar energy such as PV cells and Thermal collectors. The PV cells utilizes the photovoltaic technology to convert the sunlight into electrical energy directly with the help of semiconductors (Mehrara, M. 2007). ENN552 Solar Thermal Systems
The thermal collectors collects the thermal energy from the sunlight and utilizes the thermal energy to generate steam and to run a turbine which is coupled with electric motors and generates electricity. Some of the solar thermal collectors that are being used are parabolic trough collector, Fresnel lens collector, Heliostat field central receiver system, etc. The flat plate and the parabolic collectors are used for small and medium power generation capacities. The heliostat thermal collectors are applicable in higher power generation plants. (Boyle, G. 2004).
2.0 Power Generation Using Solar Thermal Collectors:
The Solar thermal collectors collects the heat energy from the sunlight and utilizes it to generate electricity with the help of various arrangements such as the reflective mirrors, Heat exchanger, turbine, Heat transfer Fluid, generator, etc. (the schematic of a simple solar thermal collector is shown in the Figure2.) The heat transfer fluid are allowed to flow through the tubes which are placed on the reflective mirrors, when the sunlight hits the reflective mirrors the heat energy is concentrated into the tubes which in turn supplies heat energy to the HTF. Then with the help of a heat exchanger the heat transfer takes place. Inside the heat exchanger the generation of steam is done. The generated steam is allowed to pass through the turbine. The mechanical energy obtained from turbine is converted into electrical energy with help of generators.
2.1 Heat Transfer Fluids:
The Heat Transfer Fluid is used to transfer the heat energy from the collector to the heat exchanger. Some of the variables that are considered during the selection of the HTF are its Coefficient of expansion, Viscosity, Thermal storage capacity, Freezing and Boiling points. The selection of the HTF is greatly influenced by the environmental conditions in which the power plant is about to work. For example if a plant is placed in a hot dessert area then the HTF should have high boiling point, proper viscosity and a proper thermal storage capacity.(Rached, W. 2011).
Some of the most important types of Heat Transfer Fluids are:
Air is an excellent, cheap, and affordable HTF. Air has anti freezing property and it also does not boil so it is applicable both in extremely cold and hot conditions. On the negative side it has extremely low heat carrying capacity, also there will be leakage problem in ducts. (Reddy, J. N. 2014).
Water is an excellent form of HTF as it has very high heat carrying capacity and it is cheap and affordable. On the other hand it has very high freeing point and low boiling point. Water is applicable in case of low to medium temperature range operation.(Reddy, J. N. 2014).
Oils generally have high viscosity but the heat carrying capacity of oil is lower than that of the water. They also have high specific weight of gravity. There are generally 3 types of oils such as synthetic, semi synthetic and normal.(Reddy, J. N. 2014).
The molten salts are found to have greater heat carrying capacity. They are applicable in case of super heating type of solar power generators. It words under extremely high temperatures.(Reddy, J. N. 2014).
2.2 Types Of Solar Thermal Collectors Are:
The parabolic trough utilizes a collector with parabolic cross section. The heat is concentrated at the focal point of parabola. (Herrmann, U., Kelly, B., & Price, H. 2004).
It focusing effect of Fresnel lens is utilized. In this type of collector the radiation from sun is focused to absorber from top not in bottom as in case of the parabolic collectors.
Large numbers of mirrors are arranged as a heliostat, the heliostat is nothing but a mirror which can track the sun and face on its direction always. (Noone, C. J et.el., 2012).
2.3 Comparison Of Various Types Of Solar Thermal Collectors:
|Parabolic Trough||Linear Fresnel||Heliostat field|
|Power generation capacity||<10 MW||10-20 MW||30 to 200 MW|
|Operating Temperature||150 – 300 0C||400 0C||>700 0C|
|Initial cost||Low||Low||Very High|
From the comparison table we can see that the parabolic trough solar thermal collectors are used in case of low power generation power plants. The Linear frensel collector is applicable in case of the medium power generation range, also the Fresnel is cost wise efficient and it is reliable in an acceptable level. The Heliostat field is used in case of power generation requirement in a range of 30 to 200MW and it is costly.
2.4 Working Of A Solar Thermal Power Plant:
In general a solar thermal power plant consists of the following major parts:
- Solar Thermal Collector
- Cooling tower
From the image we can see that the water is pumped from the water source to the solar thermal collector, the water gets heated as it absorbs the heat that are collected from the sun with the help of solar thermal collectors. The water converts into steam once it reached its phase change point, then the heated steam is allowed to pass through the turbine, a turbine is a mechanical arrangement of number of blades connected to a single shaft in order to convert the kinetic energy of the steam into the mechanical energy. As the steam passes through the blades of the turbine it turns the shaft, which is coupled to the generator. A generator generates electrical energy from mechanical input. The electrical energy that is obtained from the generator is then passed to the storage/ usage through the transmission lines. The steam after passing through the turbine is allowed to pass through the condenser which cools down the steam into liquid water with the help of cooling tower by losing heat to the surroundings.
The whole system works on the basis of rankine cycle also known as vapour powered cycle. If the working fluid in a cycle is a phase change material which converts the kinetic energy of the working fluid into useful work then the system is said to be following a ranking cycle. A rankine cycle is a heat engine which is used to measure the operational condition of a power plant. A rankine cycle consists of 4 processes such as pumping the water, heating or converting the water into steam, generating mechanical output, and condensing steam into water.
ENN552 Solar Thermal Systems